This invention relates to polyolefin films having certain unique properties. Specifically, it relates to essentially clear polyolefin films based on polyethylene and polypropylene that exhibit an increased oxygen transmission property as compared to conventional polypropylene films known to the art, while achieving desired stiffness and optical properties to render such films highly suitable for use in the institutional packaging of perishable items, e.g., fresh fruits, vegetables and flowers.
It is known that, for many applications, particularly packaging applications, that polypropylene is less than totally satisfactory due to a relatively high permeability to oxygen. Typically, a 0.7 mil (70 gauge) biaxially oriented film of polypropylene exhibits an oxygen transmission rate (OTR) of about 125 cc/100 sq. in./day/atmosphere. Such an OTR is too great for such applications as packaging of potato chips, most chocolate candy, processed meats and cheeses and other packaging applications where an excess of oxygen can lead to premature spoilage of the packaged product.
A great amount of effort has been expended in recent years to decrease the oxygen transmission characteristics of polypropylene. The best results to date have been obtained by metallization of the films.
There are, however, some applications in which a greater and controllable oxygen transmission rate is desired. This is particularly the case when fresh fruits, vegetables and flowers are to be packaged, as these products continue to respire after they are packaged. Absence or insufficient levels of oxygen, which occurs as the oxygen initially present in the package is consumed, leads to premature senescence and spoilage of the products. At the same time, respiration leads to a build-up of moisture in the package, which can also lead to spoilage of the product if the moisture cannot escape from the package. It is thus desirable to provide polypropylene packaging wherein the oxygen content can be replenished as necessary and from which the moisture can escape.
Conventional polypropylene films of a thickness required for most applications, whether monolayer or composite, have oxygen and moisture vapor permeability values that are not sufficient to allow the optimum oxygen and moisture levels to be maintained in a sealed package containing products of the type mentioned. This is the case for both conventional consumer packages and for industrial packaging applications.
It is known to prepare microporous films based on an opaque polymer mixture comprised of about 45% to 55% of a polypropylene homopolymer and 55% to 45% of a copolymer of propylene and ethylene containing about 2% to 5% ethylene by weight, which films have greater oxygen permeability than films made of plain polypropylene. This polymer mixture is blended into a mixture comprised of about 40 to 60% of the polymer mixture and 60 to 40% of certain inorganic filler materials and is subjected to biaxial orientation. Anderson, U.S. Pat. No. 4,842,875, teaches the use of such films in the preparation of controlled atmosphere containers for use with fresh vegetables, fruits and flowers.
Microporous films known to the art as described above function reasonably well for preparing films having increased and, to a degree, controllable oxygen and moisture vapor permeability. However, the voiding pigments employed in the prior art are of a particle size large enough and are employed in concentrations great enough to result in the formation of voids of such a size that the resultant films are almost totally opaque.
It is also known to prepare polyolefin films of high oxygen transmission rate for some applications by laminating a polyethylene film of about one mil thickness to a polypropylene film of about 0.4 to 0.5 mil thickness. The polypropylene film acts as a stiffener to give the polyethylene sufficient stiffness that it can be used as a produce wrapper. The polyethylene, which has an inherently high OTR, then acts as a high OTR sealant layer.
In work leading to this invention, it has been found that, by blending low density polyethylene and polypropylene within specific blending ratios, a film can be prepared that has a sufficiently high oxygen transmission rate (OTR) for use in packaging fresh fruits, flowers and vegetables that require a continuous replenishment of the oxygen supply in the closed package. Films having the desired properties for non-institutional, retail applications can be prepared from blends consisting essentially of about 61 to 85% by weight of isotactic polypropylene and 39 to 15% by weight of low density polyethylene. Such films also exhibit a sufficiently great stiffness to permit their use in such applications.
Films prepared with the blends just described, when cast and drawn to a typical packaging film thickness, i.e. 0.7 to 0.8 mil (70 to 80 gauge), exhibit several properties that make them desirable for the non-institutional, retail packaging of fresh produce items. They are of a stiffness required of packaging materials generally, i.e. a tensile modulus sufficient to allow the films to be handled in conventional film conversion equipment. In addition, such films exhibit an OTR greater than 200 cc/100 sq. in./day/atmosphere; a level at which the packaged product can continue to respire for a reasonable time after it has been harvested, thus assuring a reasonable shelf life for the product in non-institutional, retail markets.
While films prepared with a blend of polypropylene and low density polyethylene in the ratios set forth above exhibit the OTR and stiffness required for non-institutional retail applications, they are not so clear and haze-free as is desired for prepackaging of produce for such retail applications. For retail consumer applications, a clear film is required in order to display the product most attractively to the shopper in, e.g. a supermarket display or the like. A clear film required for consumer applications is defined as one having a haze value less than about 4% as determined by the Gardner Haze Meter.
It should be noted that for non-institutional retail applications both a high degree of stiffness and clarity of the packaging film are important attributes, since these latter properties are perceived by the consumer as being an indication of the freshness of the packaged item, i.e., a limp, hazy or opaque film conveys a negative visual image to the consumer.
It has been determined that a film having the desired oxygen transmission rate (OTR) required for the non-institutional retail packaging of fresh produce items and similar products, which also have the desired stiffness and clarity properties can be formed with blends of polypropylene and low density polyethylene, which films also contain an olefin heteropolymer containing propylene and at least one other xcex1-monoolefin. The specified weight percentages of the polypropylene, low density polyethylene and olefin heteropolymer are described and claimed in a co-pending application of Mark Thomas DeMeuse filed on Oct. 28, 1999 Ser. No. 09/427,765 and entitled Films Based on Three Component Polyolefin Blend. The subject matter of this latter application is fully incorporated by reference herein.
It should be noted that for non-institutional retail applications, the packaging films for produce and similar products are required to have an OTR of at least 200 cc/100 sq. in./day/atmosphere, a modulus of at least about 195,000 PSI, and more preferable at least 200,000 PSI, and a haze value no greater than about 4%.
While the three component polyolefin blend described and claimed in the aforementioned co-pending application meets the desired requirements for films for packaging fresh produce items in the non-institutional retail market, these latter films do not have the required OTR values necessary for institutional applications, e.g., the large-quantity packaging of produce and similar items for sale to institutions such as restaurants and hospitals. Moreover, since institutional packages generally are opened by institutional employees to either serve or use the packaged items, the consumer of the product normally does not view the package. Thus, institutional packaging films are not required to be as stiff, i.e., have as high a modulus, or as clear as non-institutional retail packages, since the consumer""s perception of freshness created by a high stiffness, high clarity package does not need to be communicated to the same degree to institutional buyers. However, it is still very desirable to employ packaging films for the institutional market that have a reasonable degree of stiffness and that are not excessively hazy or opaque. Applicant has determined that for institutional applications, clarity and stiffness can be sacrificed to some degree in order to obtain the desired OTR values.
The primary property requirement for the institutional market is an OTR value of at least 325 cc/100 sq. in/day/atmosphere at a film thickness of about 70-80 gauge. Moreover, the film optical and tensile properties desirably need to be better than films of linear low-density polyethylene (LLDPE), the material presently primarily used for these industrial applications. Typical prior art films formed of LLDPE have haze values in the range of 6-20% and a typical tensile modulus value of about 25,000 psi. However, films for the institutional packaging of fresh fruits, vegetables and flowers desirably should have a haze value no greater than about 10%, more preferably less than 8% and a tensile modulus greater than 80,000 psi, and preferably no less than about 100,000 psi, while at the same time maintaining an OTR value of at least 325. It is to such an institutional packaging film that the present invention relates.
It is an object of the invention to provide polyolefin films based on polypropylene and low density polyethylene that have the properties required to be useful in modified atmosphere packaging applications for packaging fresh vegetables, fruits, flowers and similar items for institutional applications.